Purpose of this research is to determine whether antigenic differences can be demonstrated between morphologically indistinguishable, axenically-cultured Giardia trophozoites from man, cat, and guinea pig. To do this, cultured organisms were washed and pooled, then used as antigens to immunize rabbits. These animals were periodically bled and boosted, and their sera collected and stored. An indirect hemagglutination (IHA) test was developed to determine whether the rabbits had produced anti-Giardia antibody; IHA titers of 1:1024 and higher were common. These sera will be used in upcoming comparative antigen-antibody studies.